1. Field of the Invention
The present invention generally relates to an ultrasonic imaging apparatus including a transducer probe and, more particularly, to an impedance converter having higher input/lower output impedance characteristics provided in the transducer probe.
2. Description of Related Art
In an ultrasonic imaging apparatus, an ultrasonic probe having large quantities of transducer elements to transmit and receive ultrasonic beams/echoes is connected via a signal cable to a processor unit which generates a transmission signal (also referred to as a drive pulse or exciting pulse) which is applied to the transducer elements of the ultrasonic probe and to process the reception signal (reception echo) from the transducer elements.
When such an ultrasonic imaging apparatus is in the receiving mode, the transducer elements function to receive the reflected ultrasonic echoes and convert them into electronic echo signals. Therefore, this transducer element can be regarded as a kind of signal source when it is viewed with regard to the major portion of the imaging apparatus. According to the general rule in a signal transmission path, it is better that an output impedance from the signal source is as low as possible in view of signal loss. However, an output impedance of the transducer element is relatively high, for example, 400 ohms. Therefore, the signal transmission is adversely influenced by a stray capacitance which is typically 110 pF/m on the signal transmission path, mainly in the cable. Consequently, there is a tendency such that a loss of reception signal is large and thus, the S/N (signal to noise) ratio deteriorates. A deterioration in S/N ratio causes an image quality of a displayed ultrasonic image to be deteriorated. To eliminate such a drawback, it has conventionally been desired to essentially solve the deterioration in the S/N ratio.
It is therefore an object of the present invention to provide an ultrasonic imaging apparatus which can supply echo signals having a better S/N ratio to the processor unit even if a transducer probe having a high output impedance characteristic is employed.